The present invention relates to x-ray tubes and is particularly related to an apparatus that provides an indication of a specific temperature value reached by a component or fluid in an x-ray tube system. The present invention finds particular application in conjunction with indicating the temperature reached by bearing assembly components in a rotating anode x-ray tube and will be described with particular respect thereto.
Typically, an x-ray tube includes an evacuated envelope made of metal or glass which is supported within an x-ray tube housing. The x-ray tube housing provides electrical connections to the envelope and is filled with a fluid such as oil to aid in cooling components housed within the envelope. The fluid is circulated through the housing and a heat exchanger external to the housing for removing heat from the cooling fluid. The envelope and the x-ray tube housing each include an x-ray transmissive window aligned with one another such that x-rays produced within the envelope may be directed to a patient or subject under examination.
In order to produce x-rays, the envelope houses a cathode assembly and an anode assembly. The cathode assembly includes a cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode assembly and the anode assembly. This potential causes the electrons to flow from the cathode assembly to the anode assembly through the evacuated region in the interior of the envelope. A cathode focusing cup containing the cathode filament focuses the electrons onto a small area or focal spot on a target of the anode assembly. The electron beam impinges the target with sufficient energy that x-rays are generated. Accordingly, the accelerated emitted electrons gain considerable kinetic energy before striking the target. Creation of x-rays in this manner is an inefficient process with less than one percent of the electron energy being converted into usable x-ray energy. A substantial portion of the remaining energy is released as heat acquired by the anode which is dissipated and transferred through the components in the x-ray tube. For example, some heat from the anode is radiated through the envelope while some is conducted through an anode stem to the other components.
In order to distribute the thermal loading created during the production of x-rays, a rotating anode assembly configuration has been adopted for many applications. In this configuration, the anode assembly is rotated about an axis such that the electron beam focused on a focal spot of the target impinges on a continuously rotating circular path about a peripheral edge of the target. Each portion along the circular path becomes heated to a very high temperature during the generation of x-rays and is cooled as it is rotated before returning to be struck again by the electron beam. In many high powered x-ray tube applications such as CT, the generation of x-rays often causes portions of the anode assembly to be heated to a temperature range of 1200-1400xc2x0 C., for example.
In order to provide for rotation, the anode is typically mounted via an anode stem to a rotor which is rotated by an induction motor. The rotor in turn is rotatably supported by a bearing assembly. The bearing assembly provides for a smooth rotation of the rotor and anode about its axis. The bearing assembly typically includes at least two sets of ball bearings disposed in a bearing housing. The ball bearings often consist of a ring of metal balls which are lubricated by application of lead or silver to an outer surface of each ball thereby providing support to the rotor with minimal frictional resistance.
During operation of the x-ray tube, the anode assembly is passively cooled by use of oil or other cooling fluid flowing within the housing which serves to absorb heat radiated by the anode assembly through the envelope. However, a portion of the heat radiating from the anode assembly is also absorbed by the rotor and bearing assembly. For example, heat radiated from the anode assembly has been found to subject the bearing assembly to temperatures of approximately 400xc2x0 C. in many high powered applications. Furthermore, given that the bearing assembly is enclosed by the rotor, the bearing assembly is not able to easily radiate heat to the cooling fluid contained in the housing as done by the anode assembly. In fact, some rotor and bearing assembly configurations operate as a heat sink. For these reasons, a substantial amount of heat is typically transferred into the bearing assembly and the heat is not readily dissipated.
Unfortunately, such heat transfer to the bearings may deleteriously effect the bearing performance. For instance, prolonged or excessive heating to the solid lubricant applied to each ball of a bearing can reduce the effectiveness of such lubricant. When thermal loads greater than desired are applied to the solid lubricants on the bearings, the lubricant can evaporate thereby increasing friction in the bearing and contaminating the vacuum in the envelope. The increased friction in the bearings can lead to premature bearing failure and the evaporated lubricant within the evacuated envelope can result in arcing, which also deleteriously affects x-ray tube service life.
During operation in the field it is possible, or in a life critical situation necessary, for the x-ray technician to operate an x-ray tube at operating conditions that result in x-ray tube components experiencing temperatures that exceed design specifications. In addition to field operation, various processes during manufacture of the tube, such as exhausting and seasoning the tube, also subject an x-ray tube to high thermal loads. Exhausting the tube is the process in which vacuum is drawn in the tube. The tube is operated with internal components at high temperatures while a vacuum pump is operatively attached to the tube. The rate at which gas is removed from the tube and the resulting final pressure of the tube are related to the temperature of the components, such as the anode, during exhaust. The higher the temperature of the component the more effectively the gas is removed from the tube and the lower the pressure of the tube after exhaust. The bearing temperature limit results in reducing the temperature that the components can reach during exhaust.
Seasoning also produces considerable thermal loading for various x-ray tube components. Seasoning is the process in which the tube is exposed to progressively higher voltages and power. This xe2x80x9cburn inxe2x80x9d procedure assists in making the tube more electrically stable at high voltages experienced during tube operation. During the seasoning process the anode target focal track is exposed to some of the highest temperatures that it will experience. During seasoning, the focal track of the anode outgasses and evolves gas molecules into the vacuum envelope, thereby raising the gas pressure. The evolved gasses are absorbed by a getter within the vacuum envelope.
Damage to x-ray tubes due to thermal loading greater than design specifications can result in warranty claims and decreased product performance. Knowledge regarding temperature levels reached by components in x-ray tubes during field operation, exhaust and seasoning can result in design improvements, improved quality control as well as assist with resolution of warranty claims. For these reasons, it is desirable to have an indication of exposure of x-ray tube system components to temperature thresholds during operation and manufacture. This temperature information is useful to improve tube and system design, improve quality control, resolve warranty claims and assist in determining root cause of tube failure.
The present invention is directed to an x-ray tube with a temperature log apparatus that satisfies the need to provide an observable change in a temperature indicating member which is useful to record when a component of an x-ray tube has been adequately exposed to a particular temperature threshold value. An apparatus in accordance with one embodiment applying principles of the present invention includes an x-ray tube comprising an insert that has an evacuated envelope, an anode assembly and cathode assembly. The anode assembly and cathode assembly are located in operative relationship to one another within the evacuated envelope. A temperature indicating member is located in thermally conductive contact with a component of one of the assemblies located in the insert. The temperature indicating member has a temperature sensitive characteristic which results in an observable change in the temperature indicating member in response to adequate thermal exposure to at least one temperature threshold value.
In accordance with one aspect of an apparatus applying principles of the invention, the temperature sensitive characteristic of the temperature indicating member is a change in color. In one such application of the apparatus, the change in color of the temperature indicating member is irreversible.
In accordance with another apparatus applying aspects of the present invention, the temperature sensitive characteristic of the temperature indicating member is a change in physical state of the member. This change in state results in an observable change in the shape or structure of the member.
In yet another application of an apparatus including principles of the invention, a frame supports the temperature indicating member. The frame is adapted to be located in direct thermal contact with the component of the assembly in the insert. In one implementation of such an apparatus, the frame has a recess for receiving the temperature indicating member. In a further adaptation of this apparatus, a cover retains the temperature indicating member within the recess. A particular implementation of this adaptation includes a recess that is threaded and the cover is a set screw received in the threaded recess.
In accordance with another aspect of an apparatus applying principles of the present invention, the temperature sensitive characteristic provides a first observable change in the temperature indicating member which indicates adequate exposure to a first temperature threshold value and a second observable change in the same temperature indicating member for indicating adequate exposure to a second temperature threshold value different than the first threshold temperature value.
In another application of an apparatus applying principles of the present invention, the apparatus includes a first temperature indicating member and a different second temperature indicating member. The first temperature indicating member has a temperature sensitive characteristic for indicating adequate exposure to a first temperature threshold value and the second temperature indicating member has a temperature sensitive characteristic for indicating adequate exposure to a second temperature threshold value that is different than the first threshold temperature value.
In yet another embodiment of an apparatus applying principles of the present invention, the temperature sensitive characteristic of the first temperature indicating member is a different temperature sensitive characteristic than the temperature sensitive characteristic of the second temperature indicating member.
In accordance with another aspect of the invention, a method is provided for determining whether a component within an envelope of an x-ray tube has been exposed to a temperature threshold value. The method includes securing a temperature indicating member at a selected location of a component of the x-ray tube within the evacuated envelope of the x-ray tube. The temperature indicating member has a temperature sensitive characteristic for observably indicating whether a temperature threshold value has been exceeded. Next, a voltage and current is applied to the anode assembly and cathode assembly tube such that heat is produced in the x-ray tube. The temperature indicating member is analyzed to determine whether it has had adequate exposure to a temperature threshold.
In one implementation of an apparatus applying the method principles of the present invention, the step of securing the temperature indicating member is accomplished by mounting the temperature indicating member to a frame and placing the frame in thermally conductive contact with the selected location of the component.
Analyzing the temperature indication member includes retrieving the frame retaining the temperature indicating member from within the envelope and opening the frame to expose the temperature indicating member to determine whether there has been adequate exposure to the temperature threshold value.
One advantage of the present invention is that the apparatus provides a simple and obvious indication of whether a component within the vacuum envelope of an x-ray tube has been adequately exposed to at least one threshold temperature value.